So many things are behind the Boston Celtics’ sweltering defensive start.

Better defensive rebounding. The ability to dissuade shots at the rim. Scores of similarly sized wings who are able to switch across almost every position with each other. Al Horford doing a ton of switching himself. Marcus Smart’s capacity to, as a point guard, defend like a bruising power forward. Opportunistic double-teams. And, yes, even a favorable schedule that has forced them to wage battle with only three top-10 offenses to date.

All of that factors into the Celtics leading the NBA in points allowed per 100 possessions, despite leaning heavily on Kyrie Irving, a career-long liability, and two youngsters, in sophomore Jaylen Brown and rookie Jayson Tatum.

Also at the heart of their defensive standing: Head coach Brad Stevens, and the effort he demands from each and every player.

Irving credited Boston’s head honcho for this less tactical, more motivational part of his job while talking shop with NBA.com’s David Aldridge:

“Brad’s made that very clear,” Irving said, “if the (defensive) effort’s not out there, if you’re not paying attention, if you’re not preparing the way we all should be preparing — that goes from the head coach down to the 15th guy — if you’re not preparing the way you should and perfecting your craft outside of the game, being very diligent, understanding our system, why it works, why we’re doing it, why the hell would you expect to play? He made it very simple. And I think all the guys, they understand it.”

Tatum also credited the Celtics’ effort when quizzed about their success. And while these explanations are typically written off as cliches, they hold weight when talking about this team.

Irving is a perfect example. His commitment to working on the defensive end has never been higher. He isn’t getting destroyed on as many screens. He’s not giving up on dribble penetrators without trying to turn his hips and position himself to strip the ball or just generally impede paths to the basket. He’s making better off-ball switches. He’s being smart about when he helps, and about how he chases steals while playing off the ball.

This alone doesn’t guarantee the Celtics will remain atop the NBA’s defensive ranks. We need to see how they fare against a longer string of elite offenses first. But if Stevens is able to instill a renewed sense of effort and purpose in Irving, who while 25 is also in his seventh season, it’s not unfair to bank on him keeping this squad exactly where it sits right now.